Mixing device with mixing ring having offset channels with spaced baffles

ABSTRACT

A device comprising a shaft, a front ring connected to this shaft, an end ring connected to the shaft at a distance A from the front ring, and a loose mixing ring, which is freely rotatable and located between the front ring and the end ring. The mixing ring has a first region in which its inside diameter is large enough for it to be able to overlap with a region of smaller diameter of the front ring has an adjoining region in which the mixing ring has on its inner side at least two channels ( 4   k ) which run axially parallel, at an angle in relation to the axis of the shaft or helically thereto, and also an adjoining third region in which its inside diameter is large enough for it to be able to overlap with a region of smaller diameter of the end ring and the mixing ring is so long that, in its respective end positions, the other of the two rings respectively is partially overlapped by the mixing ring. The mixing ring channels are offset in relation to each other.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2003/011371 filed Oct. 14, 2003 which claims benefit toEuropean Patent Application 02023466 filed Oct. 21, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mixing device which comprises a shaft(1), a front ring (2), which is positively and non-positively connectedto this shaft, an end ring (3), which is positively and non-positivelyconnected to the shaft at a distance A from the front ring, and a loosemixing ring (4), which is freely rotatable and can be moved back andforth between the front ring and the end ring.

2. Description of Related Art

Devices for mixing polymer melts are known from the prior art.

For instance, DE-A 100 00 938 discloses a mixing device in which anassembly of mixing rings and dividing rings joined, alternately onebehind the other has been pulled onto a rotor. The mixing rings havegrooves, the dividing rings have bores. If melt passes over from themixing grooves to the bores, it is deflected and divided and is mixed inthis way. A similar principle is disclosed in EP-A1 1 000 656, with thedifference that some of the rings of the ring assembly are freelyrotatable. Both devices have the disadvantage that, on the one hand,they are structurally complex, susceptible to wear and difficult toclean and, on the other hand, high pressures are required to force themelt through the bores.

EP-A 48590 discloses an extruder mixer comprising a rotor and a stator,both the rotor and the stator having rows of semicircular cavities. Thecavities of the rotor and stator do not lie one over the other but areoffset somewhat with respect to one another. As a result, the extrudedmaterial is not only sheared but also divided and rotated. This devicetoo is structurally complex and, furthermore, has the disadvantage thatthe mixing effect is only sufficient for practical requirements withrelatively long stators. In addition, when these stators are used in aninjection-molding machine, a non-return valve must be additionallyfitted, since the mixing device itself is not suitable for preventingthe melt from flowing back.

EP-B1 219 334 relates to a cavity transfer mixing extruder whichcomprises a stator and a rotor and is distinguished by the fact thatboth the stator and the rotor have two cavity groups. These cavitygroups are aligned with each other in the form of a parallelogram whenthey are projected onto a plane and, furthermore, are characterized bytheir angles of inclination. This mixing device is disadvantageousbecause, like the device referred to in EP-A 48590, this device isexpensive to produce and its mixing effect depends on the length of thestator.

JP 50-90117 discloses a mixing device for injection-molding machineswhich operates in the manner of a non-return valve. The mixing device inthis case comprises two rings, one being screwed onto the screw shaftand the other being arranged movably over it. Both parts have channels,which are arranged in such a way that they overlap and the melt istransferred from one channel directly into the other. The mixing effectof this device is not adequate for many applications.

In the patent EP-B1 340 B73, a mixing device for an extruder or aninjection-molding machine is described. This device has a separatemixing ring, arranged for free rotation about a rotor. The rotor isarranged in a stator. The mixing ring is distinguished by the fact thatit has mixing passages, which extend from the inner surface of thestator to the outer surface of the rotor. In other words, the mixingring has holes. The mixing device according to EP-B1 340 B73 can assumethe function of a non-return valve for injection-molding machines. Themixing device disclosed in the patent has the disadvantage that the melttransport is not ensured sufficiently well for all applications. On theother hand, the mixing device does not provide an adequately good sealwhen it is used as a non-return valve. In addition, the mixing effect isonly at its optimum when the mixing device has a certain minimum length.The minimum length is about twice the screw diameter 2D. Therefore, thismixing device cannot be adapted to every extruder or everyinjection-molding machine, but instead the machines must be specificallyconverted for this purpose, that is to say the screw must be shortened.Since the mixing device must have a certain minimum length, the meltvolume that is constantly present in the mixing device is alsorelatively great. This means that, for example when changing colors,relatively considerable time and material is required before usableparts of the new color are obtained. On account of the fact that themixing ring is provided with holes, it also has the disadvantage that itis mechanically weakened. The shorter the ring, the more acute thisdisadvantage, making it highly susceptible to wear.

SUMMARY OF THE INVENTION

It was an object of the present invention to provide a device which issuitable for mixing purposes and use in screw machines, and which doesnot have the disadvantages of the known mixing devices. In particular,it should be easy to produce. Furthermore, it should be designed in sucha way that it is as immune as possible to malfunctions, can withstandloading and undergoes little wear. A further aim was to find a devicewhich allows itself to be fitted and removed easily in very differentscrew machines, without complex adaptation of the screw machinesthemselves to the mixing device being required. The mixing device wasalso intended to meet the requirement of being easy to clean.

To achieve this object, a mixing device is proposed with thecharacterizing features:

that the front ring and the end ring in each case have at least onechannel (2 k) or (3 k), respectively, which runs axially parallel(axially), at an angle in relation to the axis of the shaft orhelically, and has in each case at least two regions of differentoutside diameters, of which the region with the smaller outside diameteris respectively located on the side facing the mixing ring, andthe mixing ring has a first region in which its inside diameter is largeenough for it to be able to overlap with the region of smaller diameterof the front ring has an adjoining region in which the mixing ring hason its inner side at least one channel (4 k) which runs axially parallel(axially), at an angle in relation to the axis of the shaft orhelically, and also an adjoining third region in which its insidediameter is large enough for it to be able to overlap with the region ofsmaller diameter of the end ring and the mixing ring is so long that, inits respective end positions, the other of the two rings (2) or (3)respectively is partially overlapped.

On account of its little resistance of wear and low susceptibility tomalfunctions, such a mixing device allows a screw machine to be operatedwith a long service life. If desired, such a mixing device can beinstalled in various screw machines in a simple manner.

Such a mixing device also allows, for example, a rapid exchange of themelt to be ensured, to be able to minimize as much as possible thewastage that occurs, for example, when changing colors. It also allowsthe color of an injection-molded part to be influenced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view partly in section of a mixing device inaccordance with this invention in the manner of a non-return valve inits closed position during the injection operation;

FIG. 2 is a view similar to FIG. 1 in the conveying position;

FIG. 2A is a view of Detail X shown in FIG. 2;

FIG. 3 is a plan view of the screw tip shown in FIGS. 1-2;

FIG. 4A is a side elevational view of the shaft shown in FIGS. 1-2;

FIG. 4B is a showing of Detail Y of FIG. 4A illustrating a modified formof shaft which has a smooth surface;

FIG. 4C is a cross-sectional view of the mixing ring shown in FIGS. 1-2;

FIG. 4D is a cross-sectional view of the front ring shown in FIGS. 1-2;

FIG. 5 is a plan view of a portion of the mixing ring shown in FIGS. 1-2and 4C; and

FIG. 6 is a schematic representation of the flow paths through themixing device of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The mixing device according to the invention in described in more detailbelow.

Shaft (1)

The shaft (1) of the mixing device according to the invention can inprinciple be freely selected and adapted to the requirements, ouch asthe type of material or amount of material to be mixed and the desiredthroughput. It may be round or have some other shape, for example beoval. It is generally round. Its diameter is likewise freely selectableand is generally adapted to the size of the screw machine in which themixing device according to the invention is to be fitted. It goeswithout saying that it would similarly be conceivable, if not preferred,to adapt the screw machine to the diameter of the shaft of the mixingdevice according to the invention. The length of the shaft likewisedepends on the requirements and is otherwise in principle freelyselectable. The surface of the shaft may be smooth in the region of thesection between the front ring (2) and the end ring (3) (FIG. 4B) orelse have at least one, for example two or three or even more,circumferential channels (1 k) on the section between the front ring andthe end ring. The number of circumferential channels (1 k) depends onthe one hand on the intended mixing effect, on the other hand on thetype and amount of material. The surface of the shaft is preferablyeither smooth or has a circumferential channel (1 k). Thecross-sectional shape of the circumferential channel or of thecircumferential channels is itself freely selectable, with forms thatare favorable for flow being preferred. For instance, it may beV-shaped, U-shaped, annular or rectangular. If more than onecircumferential channel is used, it is also conceivable for thecircumferential channels to have different cross-sectional shapes. It isparticularly preferred for all the circumferential channels to beU-shaped. If more than one circumferential channel is used, they arepreferably uniformly distributed over the surface of the shaft.

The mixing device according to the invention is preferably connected onone or in particular on both sides to two further devices. For thispurpose, the shaft is generally shaped in such a way that, at least onthe side on which the front ring is located, it has a means which makesit possible to connect the mixing device according to the invention to afurther device. The shaft preferably also has on the side on which theend ring is located a means with which the mixing device according tothe invention can be connected to an adjacent device. The connection ispreferably achieved by means of a thread. It is possible, however, toestablish the connection by welding, shrink-fitting, splining orinterlocking. Examples which may be mentioned of devices to which themixing device according to the invention can be connected are screwelements, such as the screw tip, kneading element, conveying element,shearing element, venting element or feed zone. Such devices are knownto a person skilled in the art or can be produced from aspects known toa person skilled in the art and adapted to the requirements.

It is particularly preferred for the shaft to be connected on the sideon which the front ring is located to a plasticizing screw and on theside on which the end ring is located to a screw tip (5).

Front Ring (2) and End Ring (3)

According to the invention, a front ring (2) and an end ring (3) areconnected positively and non-positively (fixedly) to the shaft. Therings (2) and (3) may in this case be connected releasably ornon-releasably to the shaft. It is also conceivable for one of the tworings to be connected releasably and one to be connected non-releasablyto the shaft. It is advantageous and preferred for both the front ringand the end ring to be releasably connected to the shaft. The rings mayconsequently be bolted, braced, splined, interlocked or pressed on. Inthis case, the front ring and the end ring may be connected to the shaftin the same way or else in different ways. In this respect, it isparticularly preferred to brace the front ring to the shaft and connectthe end ring non-releasably to the shaft.

According to the invention, the front ring and the end ring areconnected to the shaft at the distance A from each other. In this case,the distance A is the distance from the highest point of the front ringto the highest point of the end ring, i.e. the point at which theshoulder 2 s or 3 s, respectively, reaches the greatest outside diameterof 2 or 3. The distance A can in principle be freely selected, so thatit in adapted to the needs of the individual case and intendedapplication. For most applications, it is expedient for the distance Ato be relatively short, whereby the device according to the invention isparticularly well suited as the tip of a screw or can be used as anon-return valve in an injection-molding machine. For example, it may berecommendable to select a small distance A, if as little material aspossible is to be located in the mixing device at any time. According toone of the preferred embodiments, the distance A lies in the range from10 mm to 200 mm, particularly preferably in the range from 15 mm to 150ma, in particular in the range from 20 mm to 80 mm.

According to the invention, both the front ring (2) and the end ring (3)in each case have at least one channel (2 k) or (3 k), respectively.They may also have in each case more than one channel (2 k) or (3 k),for example two or three or even more, for example up to 50. In thiscase, it is possible for the front ring and the end ring to have thesame or a different number of channels (2 k) or (3 k). The channels (2k) or (3 k) may run axially parallel (axially) or at an angle inrelation to the axis of the shaft or helically, for example spirally. Itis particularly preferred for them to run axially parallel in relationto the axis of the shaft (axially). The number, cross-sectional shapeand arrangement of the channels (2 k) or (3 k) depends on theconditions. For example, a higher number may be advantageous if themixing effect is to be great. It is particularly preferred for the frontring and the end ring to have the same number of channels (2 k) and (3k) and it is preferred for the number of channels (2 k) or (3 k) to liein the range from 4 to 25. The channels preferably have across-sectional shape that is favorable for flow. They may, for example,be V-shaped, U-shaped, annular or rectangular. If more than one channelis used, it is also conceivable for the channels to have differentcross-sectional shapes. It is particularly preferred for all thechannels to be U-shaped. The depth of the channels depends on the designconditions, it being advantageous that the stability of the rings shouldnot be impaired.

The front ring (2) and the end ring (3) in each case have at least oneregion with a greater outside diameter and a second region with asmaller outside diameter. The two regions of the front ring or of theend ring may merge continuously one into the other and form a shoulder.Alternatively, they may, however, also merge in a stepped manner oneinto the other, so that between the region with the smallest outsidediameter and the region with the greatest outside diameter there areregions which have an outside diameter of a size in between. So theremay be one or two or more regions between the region with the smallestoutside diameter and the region with the greatest outside diameter. Itis particularly preferred for the two regions to merge continuously oneinto the other and to form a shoulder. In particular, the shoulder hasan angle a of 0 to 45°. The size of the greatest outside diameter andthe size of the smallest outside diameter are generally determined bythe inside diameter of the screw (screw root surface) and the insidediameter of the mixing ring (4). According to the invention, the secondregion with the smaller outside diameter is respectively located on theside facing the mixing ring. The configuration of the front ring and theend ring on the side facing away from the mixing ring is onlysignificant for the function of the mixing device according to theinvention to the extent that it must not disturb this function. Forexample, it may have a step or a shoulder. The respective averted sideof the rings (2) and (3) is expediently shaped in general in such a waythat the mixing device according to the invention can be connectedpositively and non-positively to a further device. The front ring andthe end ring may be configured in such a way that they have a differentshape and size. They may, however, also have the same size and in factthe same shape and differ only in that they are mirror-inverted.

Mixing Ring (4)

According to the invention, the mixing device according to the inventioncomprises a mixing ring (4). The latter is loose and freely rotatableand is located such that it can be moved back and forth between thefront ring and the and ring.

The mixing ring has a first region in which its inside diameter is largeenough for it to be able to overlap with the smaller diameter region ofthe front ring.

The first region of the mixing ring is adjoined by a second region inwhich the mixing ring has on its inner side at least one channel (4 k)which runs axially parallel (axially), at an angle in relation to theaxis of the shaft, or helically, for example spirally, particularlypreferably axially parallel (axially). In this region, the mixing ringmay, however, also have more than one channel (4 k). For example, it mayhave two or three or more channels (4 k), for example up to 50. Thenumber of channels (4 k) generally lies in the range from 4 to 20. It isparticularly preferred for at least one of the channels (4 k) to ranaxially parallel (axially). In particular, it is preferred for all thechannels (4 k) to run axially parallel (axially). According to one ofthe particularly preferred embodiments, at least two of the channels (4k) run axially parallel (axially) and are offset in relation to eachother. It is quite particularly preferred for all the channels (4 k) torun axially and to be offset in relation to one another in such a waythat baffle areas are produced between them. The channels generally havea U-shaped cross section, but may also have some other cross-sectionalshape and be, for example, V-shaped, annular or rectangular. Thechannels (4 k) may differ from one another in their depth and width.

The second region of the mixing ring is adjoined by a third region. Inthe third region the mixing ring has an inside diameter which is largeenough for it to be able to overlap with the smaller diameter region ofthe end ring. Depending on whether the smaller diameter region of thefront ring aid the region of the end ring are of the same size or havethe same shape, the first and third regions (i.e. the two outer regions)of the mixing ring are likewise of the same size and the same shape.Otherwise, they are different.

According to the invention, the mixing ring is so long that, in itsrespective end positions, the other of the two rings (1) or (2)respectively is partially overlapped. This means that, if one of its endfaces terminates with a positive connection with one of the two rings(2) or (3), the other of the two rings (2) or (3) respectively isoverlapped partially, to be precise in each case in its region ofsmaller diameter, respectively. The length of the mixing ring isgenerally from A minus 3% to A minus 20%. The length of the mixing ringpreferably lies in the range from A minus 6% to A minus 12. Of theoverall length of the mixing ring, the second or middle region generallytakes up from 20 to 70% and the two outer regions together take up from30 to 60%. The first and second regions may be of the same length. It isalso possible for the first and second regions to be of differentlengths. According to a preferred embodiment, the first and secondregions are of the same length.

The mixing device according to the invention is preferably used in ascrew machine, in this respect preferably in single-screw machines.According to one embodiment, the mixing ring terminates with the endring with a positive connection. This embodiment is particularlypreferred whenever the mixing device according to the invention is usedas the mixing element in an extruder. According to another embodiment,the mixing ring is displaced axially between the front ring and the endring. This embodiment is particularly preferred whenever the deviceaccording to the invention is used in the manner of a non-return valve,in particular for injection-molding machines.

The mixing device according to the invention is suitable for mixingviscous material with additives. By means of the device according to theinvention, viscous materials can be mixed by dividing them up into smallvolume units in the axial direction. The mixing device according to theinvention preferably comprises baffles, which are produced for exampleby the channels (4 k) of the mixing ring being arranged offset inrelation to one another. The mixing effect of the mixing deviceaccording to the invention is assisted by the presence of mixing zones,which have a constant size and may be enclosed by mixing zones ofvariable size. The latter may be produced by the mixing ring movingaxially. The device according to the invention can be produced easilyand at low cost and can be easily cleaned on account of the absence ofdead zones.

In particular, the mixing device according to the invention is alsosuitable for thermally sensitive viscous materials. Viscous materialsmay be understood as meaning thermoplastic polymers, the melt of whichcan be processed in a screw machine. Apart from these, however, otherviscous materials also come into consideration, for example those in thearea of building materials, in the area of natural substances, such asfood, or in the area of medicinal substances. The type and amount of theadditives depend of course on the viscous material which is to beprocessed.

According to a preferred embodiment, the mixing device according to theinvention is used for mixing melts of thermoplastic polymers, includingin particular polymers in the field of styrene copolymers, such asstyrene-acrylonitrile copolymers (SAN), acrylonitrile-butadiene-styrenecopolymers (ABS) or acrylonitrile-acrylate-styrene copolymers (ASA) withadditives, in particular colorants. In particular, the mixing device isused for this purpose in connection with a screw tip in the manner of anon-return valve with a mixing effect in an injection-molding machine.

In the case in which the mixing device according to the invention inoperated in the manner of a non-return valve in an injection-moldingmachine, during the injecting operation the mixing ring (4) initiallybears against the front ring (2) with a positive connection and arearwardly sealing effect, so that flowing back of the melt isprevented. After the beginning of the plasticizing operation, the mixingring moves toward the end ring (3) under the pressure of the meltarriving, which is conveyed by the screw and in which the colorant isstill insufficiently distributed. The rotational speed of the mixingring remains at zero until the mixing ring bears against the end ringwith a positive connection. Since the bearing surface, which may forexample be the shoulder of the end ring, is generally small, therotational speed of the mixing ring is generally much lower than therotational speed with which the injection-molding machine is operated.The melt conveyed by the screw is divided by the front ring into thenumber of channels (2 k) during the plasticizing operation. If themixing ring bears against the front ring (2) with a positive connection,the melt streams are divided directly into volume elements by openings(4 o ₁) in the mixing ring. The melt streams then impinge for exampleagainst baffles (4 p) of the mixing ring, which are produced by thechannels (4 k) of the mixing ring being arranged offset in relation toone another. The melt streams are mixed in the mixing zones (4 m),further distributed through the openings (4 o ₂) and pass at the end ofthe openings (4 o ₂) into a further mixing space (3 m). From there, themelt is divided once again by the channels (3 k) of the end ring. If themixing ring moves toward the end ring, a mixing space (2 m) is producedbetween the front ring and the mixing ring. This becomes larger as themovement of the mixing ring proceeds. At the same time, the mixing space(3 m) which lies between the end ring and the mixing ring becomessmaller and is no longer present when the mixing ring bears against theend ring (3) with a positive connection. In this case, the melt whichleaves the openings (4 o ₂) in the mixing ring is divided directly intovolume elements by the channels (3 k) of the end ring.

In principle, any screw tip which allows the melt to leave into thefront space can be used as the screw tip.

The mixing device according to the invention has the particularadvantage that its mixing effect is substantially independent of thelength of the device. Its mixing effect can be influenced in particularby the number and cross-sectional shape of the channels. In general, themixing device according to the invention has a length which does notexceed 1.8 D, but is preferably not longer than up to 1.5 D, inparticular up to 1.2 D, D being defined as the diameter of the screw.

An exemplary embodiment of the mixing device according to the inventionis schematically explained in more detail in the drawings, in which;

FIG. 1:

shows a view of a mixing device according to the invention in the mannerof a non-return valve during the injection operation (i.e. in the closedposition) in conjunction with a screw tip

-   2 front ring-   2 k channel in the front ring for distributing the melt-   3 end ring-   3 k channel in the end ring for distributing the melt-   3 m mixing space between end ring and mixing ring-   3 s shoulder, transition from the region 3 b ₂ to the region 3 b ₁,-   4 mixing ring-   4 k channel in the mixing ring for distributing the melt-   4 m mixing space in the mixing ring-   4 p baffle-   5 screw tip    FIG. 2:    shows a view of a mixing device according to the invention in the    manner of a non-return valve during the plasticizing operation (i.e.    in the conveying position) in conjunction with a screw tip-   1 shaft-   1 k channel running around the shaft-   2 m mixing space between front ring and mixing ring-   2 s shoulder, transition from the region 2 b ₂ to the region 2 b ₁-   5 screw tip-   A distance between the front ring and the end ring    FIG. 2A    Detail X of FIG. 2-   2 front ring-   4 mixing ring    FIG. 3;    shows a plan view of a screw tip which can be connected to the    device according to the invention-   5 screw tip    FIG. 4    shows a view of individual parts of a mixing device according to the    invention    FIG. 4A;    view of a shaft with screw tip-   1 shaft-   1 k channel running around the shaft-   1 h mounting device for front ring-   3 end ring (connected to screw tip)    FIG. 4B.    Detail Y of FIG. 4A, modified to show smooth surface of shaft (1)    FIG. 4C:    section through a mixing ring (4)    FIG. 4D:    section through a front ring (2)    FIG. 5    shows a plan view of a mixing ring 4 (partial view), which has    channels 4 k of different cross-sectional shapes    FIG. 6    shows a schematic representation of the dividing of the viscous    material as it passes through a mixing device according to the    invention (mixing ring is bearing against front ring, i.e. injection    operation)-   2 front ring-   2 k channel in the front ring-   4 k channel in the mixing ring-   4 m mixing zone-   4 ₀₁ opening-   4 ₀₂ opening-   4 p baffle-   3 end ring-   3 k channel in the end ring-   3 m mixing space

1. A mixing device comprising a shaft (1), a front ring (2) and an endring (3) connected to the shaft at a distance A from each other, and aloose mixing ring (4), which is freely rotatable and can be moved backand forth between the front ring and the end ring, wherein the frontring and the end ring in each case have at least one channel (2 k) or (3k), respectively, which runs axially parallel, at an angle in relationto the axis of the shaft or helically, and has in each case at least tworegions of different outside diameters, of which the region with thesmaller outside diameter is respectively located on the side facing themixing ring, and the mixing ring has a first region in which its insidediameter is large enough for it to be able to overlap with the region ofsmaller diameter of the front ring, has an adjoining second region, andalso an adjoining third region, in which its inside diameter is largeenough for it to be able to overlap with the region of smaller diameterof the end ring and the mixing ring is so long that, in its respectiveend positions, the other of the two rings (2) or (3) respectively ispartially overlapped, characterized in that the mixing ring has on itsinner side of the second region at least two channels (4 k) which runaxially parallel and are arranged offset in relation to each other, andthe mixing ring further comprising spaced baffles arranged in the atleast two channels.
 2. The device as claimed in claim 1, wherein theshaft has a smooth surface on the section between the front ring and theend ring.
 3. The device as claimed in claim 1, wherein the shaft has atleast one circumferential channel (1 k) on the section between the frontring and the end ring.
 4. The device as claimed in claim 1, wherein thedevice can be connected on both sides to an adjacent device selectedfrom the group consisting of a screw tip, a kneading element, aconveying element, a venting element and a feed zone.
 5. The use of thedevice as claimed in claim 1 as a mixing element in a screw machine. 6.The use of the device as claimed in claim 5 in connection with a screwtip (5).
 7. A screw machine, comprising at least one device as claimedin claim
 1. 8. An injection molding machine including at least onemixing device as claimed in claim
 1. 9. The device as claimed in claim1, wherein the mixing ring includes spaced baffles in all of the mixingring channels.